1. Field of the Invention
The present invention relates to an improved sole for footwear and more particularly to a sole which absorbs, stores and returns kinetic energy to a wearer of the footwear during the gait cycle.
2. Summary of the Related Art
Recently, considerable efforts have been devoted to develop improved running and other athletic shoes. Currently, there are many different types of running or athletic shoes which purport to provide cushioning from impact and comfort for all phases of activity. Shock absorption has been the primary focus of most of these research efforts. For example, U.S. Pat. No. 4,541,184 (Leighton) discloses an insole which is designed to provide shock absorption in the areas of the foot that are most subject to impact forces from ground contact.
Recent advances in biomechanics, however, indicate that cushioned running shoes may decrease the efficiency of the user. Experimenters have found that the arch of the foot acts like a spring, absorbing the energy of impact with the ground and giving it back with surprising efficiency to launch a runner forward again. Cushioned shoes, however, act to absorb the kinetic energy for the athlete. Up to 67% of the kinetic energy of a gait cycle may be absorbed and wasted by conventional athletic shoes.
The problem which must be addressed is not only how to minimize impact and provide comfort for the athlete's foot in running, jumping and other athletic endeavors, but also how to harvest and utilize energy resulting from certain phases of walking or running such as heel strike, midstance and toe off.
Some efforts have been devoted to develop devices which absorb and return a portion of the energy of the impact between a runner's foot and the ground. For example, U.S. Pat. No. 4,628,621 (Brown) discloses a rigid orthotic insert made of a plurality of layers of graphite fibers. The insert includes a mid-arch portion which is slightly raised relative to the rear portion and the forward portion of the insert. The insert however is disposed above the sole on the shoe. As discussed above, up to 67% of the gait cycle may be absorbed by cushioned soles. Therefore, most of the kinetic energy of the wearer is absorbed before reaching the orthotic insert.
U.S. Pat. No. 4,486,964 (Rudy) discloses a pair of moderators made of spring-type material which absorb and return kinetic energy. A first moderator is disposed in the heel area and absorbs high shock forces at heel strike. This moderator, which is shaped to cup and center the calcaneus at heel strike, elastically deforms and absorbs the energy at heel strike. As the athlete's gait cycle continues and the force on the moderator is reduced it returns the energy to the athlete. The second moderator disclosed by Rudy engages the forefoot of the athlete and has similar properties.
U.S. Pat. No. 5,353,523 (Kilgore et al.) has also addressed the issue of energy return. Kilgore et al. provide upper and lower plates which are separated by one or more foam columns. The foam columns, or support elements, are formed as hollow cylinders from a microcellular polyurethane elastomer whereas the upper and lower plates are formed from a semi-rigid material such as nylon, a polyester elastomer, or nylon having glass mixed therethrough. Further, within the hollow areas of the support elements are gas pressurized bladders. Kilgore et al. relies upon the use of microcellular polyurethanes to restore the energy imparted during impact and upon the two element cushioning component to provide proper cushioning to the wearer.
The devices of Rudy, Brown and Kilgore et al. do not return the impact energy to the runner during the entire gait cycle due in part to the presence of the elastomeric material forming the midsole of the shoe wich absorbs the energy.
The gait cycle typically consists of heel strike, midstance, a forward roll of the foot to the ball of the foot (toe break), and toe off. At the start of the walking gait cycle the initial part of the foot to engage the ground is the outward portion of the heel. This phase of the gait cycle is referred to as heel strike. Next the foot rolls to midstance and then rolls forward to the ball of the foot. In the final phase, referred to here as toe off, the toes propel the foot off the ground. The large toe provides the majority of the propelling thrust during this phase. It may provide up to 70% of the total thrust with the four small toes providing the balance.
The running gait cycle differs from the walking gait cycle in that the initial part of the foot to engage the ground is the outward portion of the arch rather than the heel. Ground reaction forces and the line of progression of ground reaction forces on a runner's foot have been studied by Cavanagh et al., “Ground Reaction Forces in Distance Running”, 13 J. Biomechanics 397 (1980). It would be advantageous to provide a device which utilizes the impact forces developed along the lines of progression of forces along the foot to optimally return the kinetic energy of the wearer's foot back to the wearer throughout the gait cycle during walking and/or running.
Shoe mechanics studies also provide other desirable features which advantageously use the mechanics of the gait cycle. For instance Perry et al., “Rocker Shoe as Walking Aid in Multiple Sclerosis”, 62 Arch Phys. Med. Rehabil. 59 (1981), demonstrates that clogs with a rocker bottom significantly facilitate ambulation of patients with certain neurologic deficits. The study suggests that a mean savings of 150% of normal energy was gained by multiple sclerosis patients which used a shoe having a rocker bottom sole.
Another factor which must be accounted for is the 25° external torsion of the foot and ankle relative to the knee axis in a gait cycle. That is, at toe off the foot twists outward, at an average angle of 25°, as the knee and hip extend forward.
It would be advantageous to provide a shoe which utilizes the rocker bottom principle along with the biomechanics of the gait cycle to improve the efficiency of an athlete. Such a shoe could harvest and utilize the energy resulting from certain phases of walking or running, store up the energy and return the energy to the athlete, thereby improving the efficiency of the athlete.